The subject matter disclosed herein relates generally to Magnetic Resonance Imaging (MRI), and more particularly to using MRI to determine electrical properties, such as the electrical properties of tissue.
MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between Radio Frequency (RF) waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. When a substance, such as human tissue, is subjected to the main magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with the main magnetic field. When excited by an RF wave, the spins precess about the main magnetic field at a characteristic Larmor frequency. A signal is emitted by the excited spins and processed to form an image.
A determination of tissue conductivity and permittivity are useful in estimating local RF power deposition (also known as local specific absorption rate) during MR imaging. This local distribution of RF power has come under increasing study with higher field imaging and with the use of multiple transmitters.
Tissue electrical properties may also be used during the therapeutic application of heat with RF, for example, RF hyperthermia. During the application of RF power, the temperature rise at a given location in the body is related to the tissue conductivity and the electric field strength at that location. The electric field strength and distribution in turn depend on the permittivity and conductivity values of the tissue types. Therefore, tissue electrical properties are parameters that may be used in hyperthermia treatment planning and optimization
These electrical properties may also have diagnostic value as malignant tissue types have been shown to have higher permittivity and conductivity than surrounding healthy tissue.
The electrical properties of tissue may be estimated using Electrical Impedance Tomography (EIT), which uses electrodes placed on the surface of the body. Thereafter, currents are applied to the electrodes, resulting in voltages recorded by another set of electrodes. An inverse problem is then solved to estimate the conductivity and permittivity of tissue within the body. Thus, in EIT systems, electrodes must be used, which can be uncomfortable to patients. Additionally, significant computational resources may be needed to solve the inverse problem, which adds time to the overall processing.
The estimation of tissue electrical properties may also be performed using the spatial variation of a transmit magnetic field in MRI (with B1+ maps). However, in these known methods, in addition to the transmit field (B1+), the receiver sensitivity (B1−) and the z-component of the RF magnetic field (Hz) must be determined, which are not typically available.